Saturn's moon Enceladus spews clues on habitability

A pair of papers in this week's Nature look at evidence that Enceladus, a moon of Saturn, may have an ocean of liquid water beneath its icy surface. Such an ocean would bode well for Enceladus harboring some kind of extraterrestrial life, in a location close enough for robotic probes to visit.

At the moon's south pole, plumes of water vapor and ice spew into space from cracks in the surface, leading to speculation that an ocean feeds the eruptions. A team of European researchers provides evidence to support that theory in a Nature study published online today, in which they used the Cassini spacecraft to spot salty ice grains in Saturn's E ring. (Scientific American is part of the Nature Publishing Group.)

That ring is largely produced by Enceladus's icy jets, so the presence of sodium in the E ring points to a salty ocean below the moon's surface. The sodium compounds, the authors write, "exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core."

But the second study, published concurrently, examines the vapor from the moon's plumes, which greatly outweighs the ice grains, from ground-based observatories and does not see the sodium that would be expected if the plumes were indeed geysers from a near-surface saltwater ocean. The research, the study's authors write, shows "that the bulk of mass escaping Enceladus has much lower salt content than ocean models." Evaporation from a deeper ocean might explain the lack of sodium in the vapor, as might a freshwater reservoir on Enceladus.

In a related commentary on the two studies, space scientist John Spencer of the Southwest Research Institute in Boulder, Colo., weighs the two arguments and finds that they may yet fit together. The salty ice grains, he says, would not provide enough sodium to be detected by the ground-based methods—there is too much sodium-free or sodium-poor material to dilute it. "In any case," Spencer writes, "those salty grains provide our current best smoking (or steaming) gun pointing to present-day liquid water near the surface of Enceladus."

Photo of Enceladus from the Cassini spacecraft: NASA/JPL/Space Science Institute

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